The debate over deep-sea mining has focused almost entirely on environmental impacts: how marvelous communities of life, little-known and unlike any others on Earth, would be affected by the industrialized extraction of seabed minerals. Not much attention is paid to the economics. It’s taken for granted that, whatever the ecological harms may be, the business case is sound.But what if that’s not true? Some opponents of deep-sea mining now argue that it wouldn’t just be ecologically catastrophic,...